The present invention is related to telephone equipment in general and in particular to a novel apparatus for restricting the telephone numbers which can be called from one or more telephone trunk or extension lines.
A typical telephone number comprises seven digits. The first three digits identify an exchange area in a particular locality. The last four digits identify a particular phone unit or a particular group of phone units in the exchange area having the same number. Other numbers, such as three-digit area codes, are frequently added to the seven-digit number for identifying groups of exchange areas in different parts of the country and are used in long-distance calls where direct dialing is available. Special single and multi-digit numbers are also employed for identifying special functions.
Two types of telephone transmitter-receiver units are presently in wide use. The best known and probably still the most widely used at the present time is the dial phone. A more recent development, known as multifrequency signalling, is, however, rapidly replacing the dial equipment in many areas. In dial equipment, there is generated a series of voltage pulses for each digit of a number dialed. If the number seven is dialed, for example, seven pulses are generated. If the number five is dialed, five pulses are generated, etc. In multifrequency signalling equipment, on the other hand, instead of voltage pulses, multi-frequency a.c. signals are generated for each digit as each multifrequency signalling digit button is depressed.
Business and professional offices each year are employing an ever increasing number of phone units for the conduct of business. While a great number of these phones are usually connected to a main telephone system through a local operator-controlled telephone switchboard, the operators of these boards are, due to heavy work loads, not in a position to exercise a great deal of control over the calls made. Also, with the advent of direct dialing, as those terms are used in connection with both dial as well as multifrequency signalling equipment, the use of local switchboards is frequently no longer required and has been dispensed with by many.
The elimination of switchboards and the reduced responsibility of operators in checking calls, while generally improving telephone service, has permitted an ever increasing rise in the amount of use of business phones for unauthorized calls. Such calls, for example, may be placed by employees, as well as by members of the public who may have been given authorization to use the phone for "local" calls, but frequently use the phones for toll calls as well. This costly situation has been found by many to be nearly impossible to control. Either for technical reasons or in order to maintain good public relations, businesses are, therefore, prone to use some sort of toll restrictor equipment which automatically restricts the calls or telephone numbers which can be called from a particular phone.
Several automatic call restrictor systems have been proposed and a number are presently commercially available. all such known systems, however, are generally large and cumbersome, complex and expensive, and not easily modified to quickly and inexpensively accept changes in the acceptable numbers which can be called. Many known systems are also severely restricted in the number of digits that can be used for checking the acceptability of telephone numbers.